This invention relates to improvements in data display systems which are adapted for use with radar indicators and video storage devices therefor of the type wherein data is received at a relatively low rate and displayed at a different rate, generally many times faster, to obtain a relatively non-flickering steady high resolution image and where the high resolution image has multiple intensity levels. The invention has particular application in weather radar systems and radar navigation systems.
It has long been a desirable feature that the indicator for the above type radars provide a continuous bright display of video information which will appear as a moving map to the pilot. This desirable feature has been closely approximated in the past by storage tube indicators. Storage tubes, however, require careful design for stable operation. The small spacing and relatively high voltage existing between the storage mesh and the viewing phosphor are vulnerable to breakdown. Operation of the tube must be controlled to prevent a bistable condition on the storage surface. In this condition, excessive beam writing can shift the storage mesh potentials sufficiently to result in secondary emission, causing a runaway condition and resulting destruction of the viewing screen mesh.
A digital display system which provided the desired continuous bright display of video information was described in the above mentioned prior related patent application. It was explained in that application that a radar return signal was converted into a train of binary bits and the bits were temporarily stored in an input buffer at a rate determined by the desired radar range. The bits were subsequently entered from the input buffer into a circulating memory where the bits were stored. The memorized bits were optionally integrated to eliminate extraneous signals, such as those caused by noise or other interference. An output buffer was provided between the memory and the cathode ray tube to permit selected portions of the memorized bits to be displayed in an ordered sequence on the cathode ray tube at a rate which may differ from the rate at which bits were entered into the input buffer. This method of sweep generation and storage permits the quality of the display to be independent of the pulse repetition frequency of the system. Therefore, a minimum pulse repetition frequency can optionally be used and still retain a relatively bright, continuous display. As a result, the average power consumed by the transmitter portion of the radar system could be much less than that required by systems having a higher pulse repetition frequency.
In addition to the above described elements, means were described for synchronizing the operation of the input and output buffers and their associated gates with the radar antenna position.
In the system described in the previous application, there was no transformation of the coordinate system. The input data from the radar receiver was in the rho-theta coordinate system and the resulting display was in the rho-theta coordinate system. However, the rates of input and output data were optimally independent.
The digital display system described in the aforementioned prior patent application has provisions for storing and subsequently displaying the information contained in a single train of serial binary bits. In other words, the resultant display was merely a twolevel display wherein one level corresponded to a logical 0 or no radar return or return below a certain threshold and a second level corresponded to a logical 1 or a radar return above the threshold.
It has been found useful to have multiple intensity levels (multilevel) displays, especially for weather radar. For example, a four level display might have a first level at which no radar return is received indicating the radar transmission has not intercepted inclement weather conditions. A second level might indicate that the returned radar signal exceeds a first threshold indicating that minor inclement weather conditions have been intercepted by the radar transmission. In like manner, third and fourth levels might indicate that successively severe weather conditions have been intercepted by the radar transmission. Using this type of weather radar there can be presented on the radar display areas of intense storm condition, called storm cells, with areas of lesser storm activity between the storm cells. An airplane pilot having this type of weather radar would then be able to maneuver his aircraft through these areas of lesser storm intensity. Of course, if a display system having only two levels were used, the pilot would be unable to distinguish areas of lesser storm activity from storm cells to the detriment of his maneuvering ability.